In order to suppress emission of carbon dioxide gas from an automobile, reduction of weight of an automobile body is promoted by using a high-strength steel sheet. Further, in order to ensure safety of passengers, the high-strength steel sheet has become widely used, in addition to a soft steel sheet, for the automobile body. Furthermore, in order to promote reduction of weight of the automobile body in future, it is necessary to increase the strength level of the high-strength steel sheet more than before, but the increase in strength of the steel sheet is generally accompanied by deterioration of material properties such as formability (workability). Therefore, how the strength is increased without deteriorating the material properties is important in developing the high-strength steel sheet. Particularly, a steel sheet used as a material of automobile members such as an inner sheet member, a structure member, and an underbody member is required to have stretch flange workability, burring workability, ductility, fatigue durability, impact resistance, corrosion resistance, and so on according to its usage. It is important how these material properties and high strength property are ensured in a high-dimensional and well-balanced manner.
Further, the steel sheet used as the material of those members needs to be improved also in low-temperature toughness so as to be resistant to destruction even when being subjected to impact caused by collision or the like after they are attached to the automobile as members after molding, particularly to secure the impact resistance in a cold district. This low-temperature toughness is defined by vTrs (Charpy fracture appearance transition temperature) or the like. For this reason, it is also necessary to consider the impact resistance itself of the above-described steel sheet.
That is, the steel sheet used as the material of parts including the above-described members is required to have the low-temperature toughness as a very important characteristic, in addition to excellent workability.
As for the improving method of the low-temperature toughness in the high-strength steel sheet, its manufacturing methods are disclosed, for example, in Patent Documents 1, 2, in which the low-temperature toughness is improved by a method including a martensite phase adjusted in aspect ratio as the main phase (Patent Document 1), and a method of finely precipitating carbide in ferrite having an average grain diameter of 5 to 10 μm (Patent Document 2).
However, in Patent Documents 1 and 2, nothing is mentioned about the stretch flangeability and poor forming may be caused when applying the steel sheet to a member that is to be subjected to burring. Further, also in a steel pipe field and a thick plate field, there is knowledge about improvement of the low-temperature toughness but the formability as high as that of a thin plate is not required, and there is similar concern.
As for the improving method of the stretch flangeability in the high-strength steel sheet, a metal structure control method of a steel sheet for improving local ductility is also disclosed, and that controlling inclusions, making a single structure, and reducing the hardness difference among structures are effective for the bendability and the stretch flangeability is disclosed in Non-Patent Document 1. Further, a technique of improving the strength, the ductility and the stretch flangeability by controlling the finishing temperature of hot rolling, and the reduction ratio and the temperature range of finish rolling, to promote the recrystallization of austenite, suppressing development of a rolled texture, and randomizing the crystal orientations is disclosed in Non-Patent Document 2.
It is conceivable to be able to improve the stretch flangeability by uniformizing the metal structure and the rolled texture from Non-Patent Documents 1, 2 in which, however, no consideration is made for compatibility between the low-temperature toughness and the stretch flangeability.
For compatibility between the stretch flangeability and the low-temperature toughness is mentioned in Patent Document 3 which discloses a technology of dispersing appropriate amounts of retained austenite and bainite in a ferrite phase with controlled hardness and grain diameter. However, it is a structure containing soft ferrite at 50% or more and is thus difficult to respond to the demand for higher strength in recent years.